Field Characterization and Compensation of Vibrational Nonuniformity for a 256-Element Focused Ultrasound Phased Array

Multielement focused ultrasound phased arrays have been used in therapeutic applications to treat large tissue volumes by electronic steering of the focus, to target multiple simultaneous foci, and to correct aberration caused by inhomogeneous tissue pathways. There is an increasing interest in using arrays to generate more complex beam shapes and corresponding acoustic radiation force patterns for manipulation of particles such as kidney stones. Toward this end, experimental and computational tools are needed to enable accurate delivery of desired transducer vibrations and corresponding ultrasound fields. The purpose of this paper was to characterize the vibrations of a 256-element array at 1.5 MHz, implement strategies to compensate for variability, and test the ability to generate specified vortex beams that are relevant to particle manipulation. The characterization of the array output was performed in water using both element-by-element measurements at the focus of the array and holography measurements for which all the elements were excited simultaneously. Both methods were used to quantify each element's output so that the power of each element could be equalized. Vortex beams generated using both compensation strategies were measured and compared to the Rayleigh integral simulations of fields generated by an idealized array based on the manufacturer's specifications. Although both approaches improved beam axisymmetry, compensation based on holography measurements had half the error relative to the simulation results in comparison to the element-by-element method.